RE: XPS 18 Tablet not charging while plugged in??
With over a hundred of these XPS Tablets in the field, I have never seen a problem due to the stock charger not being able to source enough power. We have never had to uprate a brick, and have only had a couple of faults with the bricks. Of those, one or maybe two were problems with the LED at the end of the cord becoming intermittent, and the brick still worked fine. We think we had a brick fail completely, but it was before I started working with them. I would have broken the failed one open if I had access to it, I like to autopsy failures. I repaired a brick for a LaCie external USB HDD just this weekend! Not to say your scenario couldn't happen, but the stock power brick has more than enough power to do the job. I would still disconnect any devices that could be pulling power from the ports to see if it makes a difference. Trying another stock brick would certainly be one of the first things I would try if I had access to one. They are rather expensive to obtain just for a test.
I have seen bad XPS charge ports that would charge up part way overnight if left plugged in and turned off. My lead has had at least one XPS start to charge normally after being left plugged in and turned off for a day or longer. I have not had such luck. When the port just fails to recognize the charger, it will run off of the charger but will not charge the battery. You can usually simulate this failure by going into diagnostic mode and pulling the power connector out a very small distance until it stops charging. Once it can't read the charger's data over the center pin, it will drop into zero charge mode but will still be powered from the inner and outer ring on the plug.
My belief is that in some cases the charging control circuit will allow a trickle of charge current when the XPS is off. It may be related to the condition of the battery as determined by the charge controller, or it just might let a little charge current flow on some units when they are off, but not when they are on, or not enough to notice when they are on.
I doubt the stand will fix this problem if it does not charge through either port, but I have read here where some folks have had success with a different stand. Each of the ports can fail and not affect the other, so there is hope the stand port can be made to work even if the charge port will not.